The development of drugs effective against human cancers has been recognized as a critical need. This is particularly true for cancers attacking the central nervous system (CNS). Development of drugs having activity against CNS specific cancer cell lines which are effective in vivo has been especially difficult due to the inability of many anticancer drugs to penetrate the blood-brain barrier or the blood-cerebrospinal fluid (CSF) barrier.
The blood-brain barrier, existing between the blood and brain fluid, and the blood-CSF barrier, existing between the blood and CSF, prevent many large molecular substances from passing from the blood to the interstitial fluids of the brain and the CSF. Generally, these barriers are highly permeable to water, carbon dioxide, oxygen and lipid soluble substances and slightly permeable to electrolytes.
As recognized, the delivery of drug species to the brain is often seriously limited or inhibited by the functional barrier of the endothelial brain capillary wall deemed the blood-brain barrier, BBB. The barriers separating plasma from the brain and cerebrospinal fluid (CSF) are complex systems involving passive and active transport and subserve a number of important functions. The BBB is, moreover, basically the result of the fact that the endothelial cells in the brain capillaries are joined by continuous, tight intercellular junctions, such that material has to pass through the cells rather than between them in order to move from blood to brain.
Thus, foreign compounds, such as therapeutics, which enter organs other than the central nervous system with ease, may penetrate the CNS slowly or hardly at all. A number of theories concerning the nature of the barrier have been proposed. One theory is that the boundary is essentially a fat-like layer interspersed with small pores, although the BBB is not a simple, anatomically well-defined unitary physical entity. Penetration of such a barrier may occur by several processes: lipid soluble substances may passively penetrate into the cells, while small molecules such as water and urea may pass through the pores.
In addition to the aforementioned simplified physical processes, carrier-mediated and active transport processes act to govern the movement of many molecules through the BBB. Some materials such as glucose and amino acids are transported by an active mechanism, characterized by saturation, bidirectional molecular specificity, bidirectional competitive inhibition and bidirectional countertransport. Fishman, Am. J. Physiol., 206, 836 (1964).
It is also known that the BBB is relatively impermeable to the ionized forms of drugs and other molecules. Drugs which are weak organic electrolytes appear to pass from blood to BBB to reach a steady state ratio characteristic of each molecule according to its pKa and the existence of a normal pH gradient between blood and BBB. It is clear, therefore, that quaternary nitrogen-containing salts, such as pyridinium or ammonium salts, penetrate the BBB only with great difficulty, if at all.
Chemical delivery systems (CDSs) have been used to deliver drugs to particular organs. These CDSs generally require that a biologically inert molecule be covalently attached to a drug, thereby producing a highly lipophilic conjugate which can easily penetrate the blood-brain barrier. Several chemical and/or enzymatic steps are then required to release the active drugs. A principal carrier for such CDSs that are used to cross the blood-brain barrier is the dihydropyridine moiety obtained from Na.sub.2 S.sub.2 O.sub.4 reduction of the appropriately substituted pyridinium salt.
U.S. Pat. No. 4,479,932 relates to certain CDSs that tether the target drug species to a reduced, blood-brain barrier penetrating lipoidal form of a dihydropyridine-pyridinium salt type redox carrier. Oxidation of the dihydropyridine carrier moiety in vivo to the ionic pyridinium salt type drug/carrier entity prevents elimination thereof from the brain, while elimination from the general circulation is accelerated, and subsequent cleavage of the quaternary carrier/drug species results in sustained delivery of the drug in the brain and facile elimination of the carrier moiety.
Pyridocarbazole alkaloids such as ellipticine, i.e., 5, 11-dimethyl-6H-pyrido [4,3-b] carbazole (i.e., R=H in the following general formula) and 9-methoxyellipticine (i.e., R=OCH.sub.3 in the following general formula) are known as alkaloids contained in, for example, Aspidospermina and Ochrosia leaves. ##STR1##
Further, U.S. Pat. No. 4,045,565 discloses that 9-hydroxy-5,11-dimethyl-(6H)-pyrido [4,3-b] carbazole having the following formula: ##STR2## has considerably high antitumor properties with respect to many tumors and particularly mouse L-1210 leukemia.
It has also been reported by J. Rouesse et al., Bull. Cancer (Paris), 68, 437-441 (1982) that 2-methyl-9-hydroxyellipticinium acetate (Celiptium) having the formula: ##STR3## is effective against mammary cancer. It was also reported, by R. W. Guthrie et. al, J. Medicinal Chemistry, 18 (7), 755-760 (1975), that ellipticine and 9-methoxyellipticine are effective against the tumor of animals used for experiments, mouse lymphoid leukemia L-1210 and Sarcoma 180 (solid) and, in Japanese Examined Patent Publication (Kokoku) No. 58-35196 and British Pat. No. 1436080, that the activity of 9-hydroxyellipticine against mouse lymphoid leukemia L-1210 is higher, by more than 100 to 1000 fold, than that of 9-methoxyellipticine.
Ellipticine compounds have been found to have anti-cancer activity, including elliptinium which exhibits in vitro activity against human glioma lines (SF126, SF375 and SF407), ellipticine glycosides which are active against L1210, P388, B16 melanoma and colon carcinoma in vivo, carbamate which is active against various human lung cancer lines, datelliptium (2-[(2-diethylamino) ethyl]-9-hydroxyellipticinium) which has in vivo activity against L1210, P388, B16, colon and M5076 reticulosarcoma, pazellipticine (PZE) which has in vitro activity against L1210 cells, and oxazolopyridocarbazoles which have antitumor activity in vitro. The mechanism for the antitumor activity of these compounds is unclear. Important factors include metabolic activation of an ellipticine to a quinone imine or related species of high electrophilicity, DNA intercalation, and topoisomerase II as a critical cellular target.
U.S. Pat. No. 4,698,423 also relates to certain quaternary ellipticine derivatives that have antineoplastic or antitumor activity. In accordance with that patented invention, there is provided an ellipticine derivative having the general formula: ##STR4## wherein,
R.sub.1 represents a hydrogen atom, a hydroxyl group, an alkoxyl group having 1 to 4 carbon atoms, or an acyloxy group having 2 to 7 carbon atoms;
R.sub.2 represents an aldose residue, a deoxyaldose residue, an N-acylaminoaldose residue, an aldohexuronic amide residue, an aldohexuronic acid residue, an acylated aldose residue, an acylated deoxyaldose residue, an acylated N-acylamino aldose residue, an acylated aldohexuronic amide residue, an acylated aldohexuronic acid residue, an acylated aldohexuronic acid ester residue, an arylalkylated aldose residue, an arylalkylated N-acylaminoaldose residue, an arylalkylated aldohexuronic amide residue, an arylalkylated aldohexuronic acid residue, an arylalkylated aldohexuronic acid ester residue; and
R.sub.3 represents a hydrogen atom, a linear, branched, cyclic, or cyclic-linear alkyl group having 1 to 5 carbon atoms;
X.sup.- represents a pharmaceutically acceptable inorganic or organic acid anion; and
The bond represented by N.sup.+ -R.sub.3 in the general formula represents a glycoside bond between a nitrogen atom in the 2-position of the ellipticine and a carbon atom in the 1-position of the sugar.
As disclosed in U.S. Pat. No. 4,310,667, certain specific compounds are known to possess anti-cancer activity, including, the water soluble 2-N quaternary salts of the formula: ##STR5## wherein R.sub.1 is selected from the groups consisting of hydrogen and alkyl groups; R.sub.2 is selected from the groups consisting of hydrogen, alkyl and acyl groups; R.sub.3 is an alkyl group; and X.sup.- is an anion.
The problem with the aforementioned ellipticinium derivatives is that the quaternary salt nature of the anti-cancer compounds limits them to only act peripherally because they are unable to cross the blood brain barrier. As a result, these compounds are ineffective against CNS specific cancer cell lines.
A need has therefore existed for compounds and pharmaceutical compositions having activity against CNS specific cancer cell lines which can easily penetrate the blood brain barrier without the need for a separate CDS.